docs: def get_true_longitude_variables(nc):
      '''
      Returns a list of variables defining true longitude.
  
      CF Chapter 4 refers to longitude as a coordinate variable that can also be
      used in non-standard coordinate systems like rotated pole and other
      projections. Chapter 5 refers to a concept of true longitude where the
      variabe defines longitude in a standard projection.
  
      True longitude, for lack of a better definition, is simply longitude where
      the standard_name is longitude or the units are degrees_north.
  
      :param netCDF4.Dataset nc: An open netCDF dataset
      '''
      lons = get_longitude_variables(nc)
      true_lons = []
      for lon in lons:
          standard_name = getattr(nc.variables[lon], "standard_name", None)
          units = getattr(nc.variables[lon], "units", None)
          if standard_name == 'longitude':
              true_lons.append(lon)
          elif isinstance(units, basestring) and units.lower() in VALID_LON_UNITS:
              true_lons.append(lon)
      return true_lons